1. Technical Field
The present invention relates to an organic electroluminescence element (hereinafter also referred to as the “organic EL element”) which utilizes the electroluminescence (hereinafter also referred to as the “EL”) of organic compounds which emit light in response to a current injected thereinto, and has a light emitting layer formed of a laminate of such materials.
2. Description of the Related Art
Generally, each of the organic EL elements constituting a display panel using organic materials comprises an anode as a transparent electrode, a plurality of organic material layers including an organic light emitting layer, and a cathode comprised of a metal electrode, which are laminated as thin films in this order on a glass substrate as a display surface. The organic material layers include, in addition to the organic light emitting layer, a layer of a material having the hole transport capability such as a hole injection layer and a hole transport layer, a layer of a material having the electron transport capability such as an electron transport layer and an electron injection layer, and so on. Organic EL elements comprising these layers are also proposed. The electron injecting layer also contains an inorganic compound.
As an electric field is applied to the laminate organic EL element including an organic light emitting layer and an electron or hole transport layer, the holes are injected from the anode, while electrons are injected from the cathode. The electrons and the holes are recombined in the organic light emitting layer to form excitons. The organic EL element utilizes light which is emitted when the excitons return to a base state. In some cases, a pigment may be doped into the light emitting layer for improving the efficiency of light emission and stably driving the element.
Recently, it is suggested that phosphorescent materials are used for the light emitting layer of the organic EL element in addition to fluorescent materials.
It is expected that the ratio of production of singlet and triplet excited states is 1:3 because the EL element using phosphorescence may achieve an high light efficiency as three or four times of the device using fluorescence.
On the other hand, for improving the low power consumption nature, light emission efficiency, and driving stability of the organic EL element, it has been proposed to provide a hole blocking layer between the organic light emitting layer and the cathode for limiting the migration of holes from the organic light emitting layer. Efficient accumulation of holes in the light emitting layer with the aid of the hole blocking layer can result in an improved recombination probability with electrons, and a higher light emission efficiency. A report has been made that single use of a phenanthroline derivative or a triazole derivative is effective as a hole blocking material (see Japanese Patent Application Kokai Nos. Hei 8-109373 and Hei 10-233284).
There is an organic EL element utilizing emission of phosphorescence. The conventional organic EL element comprises a light emitting layer including a material having a hole transport capability as a host material; and a hole blocking layer disposed at the side of the cathode adjacent to the light emitting layer and having an ionization potential energy higher than that of the light emitting layer's host material, for example, a phenanthroline derivative such as 2,9-dimethyl-4,7-diphenyl-,1,10-phenathroline: BCP, or an aluminum chelate such as ((1,1′-biphenyl)-4-olato)bis(2-methyl-8-quinolinolato Nl, O8) aluminum: BAlq. When BCP is used for the hole blocking layer, there is a drawback in that the driving lifetime of the organic EL element is very short although the early emission property thereof is good. There is no material for the hole blocking layer having both of an excellent durability and a high ionization potential energy enough to block holes in the actual situation at the present time.
Although BAlq has an excellent durability but not a sufficient hole blocking capability because of a less than a high ionization potential energy. In fact, when an organic EL element utilizing the hole blocking layer of BAlq and the electron transport layer of tris(8-hydroxyquinolato Nl, O8) aluminum: Alq3 is driven, the electron transport layer emits green light. If such an organic EL element utilizes phosphorescence for red emission, the green light emitted from Alq3 influences chromatic deterioration itself so that red emission changes into orange.
While the provisions of an organic phosphorescent material light emitting layer and a hole blocking layer is effective for increasing the light emission efficiency of the organic EL element, a longer lifetime of the element is required. There is a need for a highly efficient organic electroluminescence element which continuously emits light at a high luminance with a less current.